Bone mill including a base and a mill head separate from the base, the base including a biasing member that urges the mill head away from the base

ABSTRACT

A bone mill including a base and a mill head that is removably attached to the base. A cutting disc is moveably disposed in the mill head. A motor is disposed in the base. A spindle attached to the motor releasably engages the cutting disk. The spindle is also capable of movement towards and away from the motor. A spring holds the mill head away from the motor. In some versions of the invention, the spring urges the spindle towards the cutting disc. The biasing force exerted by the spring urges the spindle into engagement with the cutting disc. A locking assembly releasably holds the mill head to the base in opposition of the force exerted by the spring.

FIELD OF THE INVENTION

This application is a divisional of U.S. patent application No.13/177,589, filed 7 Jul. 2011 now U.S. Pat. No. 8,343,156. patentapplication Ser. No. 13/177,589 is a divisional of U.S. patentapplication Ser. No. 11/936,448 filed 7 Nov. 2007 now U.S. Pat. No.8,002,774. The applications from which this application now claimspriority are incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates generally to a bone mill used to reduce the sizeof bone used in surgical procedures. More particularly, this inventionis generally related to a bone mill that includes a base and a mill unitthat is removably attached to the base.

BACKGROUND OF THE INVENTION

A bone mill is a medical device that, as its name implies, reduces thesize of section of bone. The milled bone, which is often chip sized, isused in a medical procedure as a filler adjacent other sections of bone.For example, in a spinal fusion procedure, it is a known practice toplace a compound formed out of milled bone around the rods used to holdadjacent vertebra in alignment. This compound serves as a lattice uponwhich the tissues forming the vertebra grow so as to form a bone-linkbetween these vertebras. This link minimizes the load imposed on therods. Milled bone is similarly used as filler and/or growth formationlattice in orthopedic surgical procedures and other procedures such asmaxillofacial procedures.

Milled bone is used as a filler/growth formation lattice in theseprocedures because the material, the proteins from which it is formedcan serve as make-up material from which the blast cells of the adjacentliving bone cells can form new bone. Accordingly, in a surgicalprocedure in which it is desirable to foster the growth of new bone,milled bone, to which supplemental material is sometimes added, isemployed as filler in the spaces in which bone growth is desired.

Milled bone is formed by taking a large mass of bone, a mass that mayhave a volume of 8 cm³ or more, and reducing it into chips. A chiptypically occupies of volume of 0.008 cm³ or smaller.

The bone mill is the device used to mill, morsellize, the large mass ofbone to chip size. A typical bone mill includes a housing. A bladeassembly or mill head is rotatably mounted to the housing. This bladeassembly/mill head is formed with cutting surfaces able to shave orbreak up the bone pressed against it. There is also some type of devicefor driving the blade assembly/mill head. If the bone mill is manuallyoperated, the drive device is often a handle capable of rotating theblade assembly/mill. A powered bone mill includes a motor that performsthis function.

A bone mill, it should also be understood, is typically designed to beused in the operating room in which the procedure is being performed forwhich the bone chips are required. This is because, in many situations,the bone used to form the bone chips comes from another portion of thepatient's body. The use of the patient's own tissue reduces thelikelihood of its rejection by the body. Therefore, an initial part ofprocedure in which the bone compound is used often involves harvesting asmall piece of bone from another portion of the patient's body. Thisbone is milled into the compound-forming bone chips. This bone harvestedfrom the patient is referred to as autograft bone. Bone from a sourceother than the patient is referred to as allograft bone.

Ideally, the compound-forming bone chips should undergo as minimal aspossible surface oxidation prior to implantation in the body. Thissurface oxidation results reduces the extent to which the materialforming the bone chips can serve as lattice or feedstock that fostersthe growth of new bone. Thus, even when allograft bone is milled to formbone chips, this milling process is performed as close as possible tothe time at which the chip compound is needed.

Known bone mills work reasonably well. Nevertheless, there are somelimitations associated with these devices. Some bone mills, for exampleare provided with reusable blades/mill heads. One disadvantage of thistype of assembly is that after each se, time must be spent todisassembly its components for cleaning and then reassemble them forlater use. A further disadvantage of this type of device is that ablade/mill head typically has a number of closely spaced apart surfacessome of which have sharp edges. After each use of the device, care mustbe taken to carefully sterilize the blade. This process can take anappreciable amount of time in order to ensure both that the blade isproperly sterilized and the person perform this task does not cuthis/her fingers on the sharp edges.

Furthermore, over time, the blades of a reusable mill invariably dull.This requires one to either by a new blade set or resharpen the existingblades.

To avoid the difficulties associated with sterilizing a bone mill blade,bone mills with use once, replaceable, mill units are available. Thistype of device includes a base to which a mill unit is removablyattached. The mill unit includes a body in which a blade is rotatablymounted. Often the base includes a motor for driving the blade. Thistype of device is designed so that, after a single use, the mill unit isdiscarded. An advantage of this type of device is that medical personneldo not have to concern themselves with sterilizing, the blade, a sharpmetal object.

Some of these systems are designed so that once the bone is morsellized,the medical personnel have to use either instruments or their fingers toremove the bone from around the blade. Having to perform this step addsto the overall time it takes to provide the bone. Some known disposablebone mills are designed so that in order to access the bone chips, themedical personnel have to remove the blade. In this step of theprocedure, personnel have to concern themselves with not getting cut bya sharp metal object.

Further, many known bone mills are constructed so that the blade mayrepeatedly strike against the same surface of the bone or bone chip. Thefrictional heat generated by such activity can damage the materialforming the bone. This damage can adversely affect the ability of thebone chips to function as growth material for the new bone.

Still another disadvantage of some known bone mills is that frequencywith which the bone milled is reduced in size in the milling processvaries within a single milling operation. Some bone may be subjected toonly nominal milling. The chips formed as a consequence of this millingmay be tool large to be used in the subsequent procedure. Still otherbone, in the same milling operation, may, as a result of repetitivemilling, be milled to down to very small, almost dust sized particles.The small size of these chips makes their collection for use in theprocedure difficult.

Moreover, a sizeable fraction of the bone milled by some bone mills isoften not easily accessible for use. This is especially the situationwhen autograft bone is used to form the bone chips. This particular bonetends to be moist. Consequently, the bone chips have been known toadhere to surfaces of the bone mill, including the actual blade. Toensure that enough bone chips formed from freshly harvested bone isavailable for use, medical personnel may sometimes feel obligated toform an excess amount of chips knowing that some will not be availablefor use. Alternatively, these personnel may, once the milling process iscompleted, have to carefully remove the adhered bone from the surfacesof the mill including the surfaces around the sharp cutting edges of theblade.

SUMMARY OF THE INVENTION

This invention is directed to a new and useful bone mill. The bone millof this invention includes a base and a mill head that is removablyattached to the base. Internal to the base is a motor. Internal to themill head is rotating cutting disc. Mounted to the mill head is amovable plunger. Also attached to the mill head and aligned with plungeris a catch tray.

To use the bone mill of this invention, the bone mill head is mounted tothe base. The motor rotates the cutting disc. The plunger is depressedto press the bone against the cutting disc. As a consequence of therotation of the cutting disc, the disc cuts the bone so as to formchips. The chips fall into the catch tray. The chips are then readilyaccessible for use by removal of the catch tray.

The mill head of this invention has features that minimize the extent towhich the formed bone chips adhere to the cutting disc and the othersurfaces of the mill head. These features thus ensure, to a significantextent that the chips formed by the mill head enter the catch tray sothey can be used in the procedure.

Still another feature of the bone mill of this invention is that once achip is cut from the bone, it does not again strike the cutting disc.This minimal contact with the cutting disc reduces the likelihood thatfrictional heat such contact can generate will damage the chip.

Further, the base and mill head are constructed so that the seating ofthe head to the base results in the auto engagement of the cutting discto the drive spindle integral with the motor. This engagement occurseven if the disc is not precisely aligned with the spindle. This featureof the bone mill of this invention contributes to minimizing the costsassociated with providing the mill head.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is pointed out with particularity in the claims. The aboveand further features and benefits of the bone mill of this invention arebetter understood by the following Detailed Description taken inconjunction with the accompanying drawings in which:

FIG. 1 is a perspective view of a bone mill of this invention;

FIG. 2 is an exploded view of the mill base;

FIG. 3 is a cross sectional view of the base leg;

FIG. 4 is a cross sectional view of the base;

FIG. 5 is a perspective view of the top of the base;

FIG. 6 is a cross sectional view of a retention arm used to releasablyhold the mill head to the base;

FIG. 7 is an exploded view of the gear train internal to the base;

FIG. 7A is a cross sectional view of the housing, the ring gear, of thegear train;

FIG. 8 is a cross section view of the gear train;

FIG. 9 is a cross sectional view of the drive coupler of the gear train

FIG. 10 is an exploded view illustrating the relationship of the spindleto the gear train;

FIG. 11 is an exploded view of the mill head;

FIG. 12 is a perspective view of the top shell of the mill head housing;

FIG. 13 is a cross sectional view of the top shell;

FIG. 14 is bottom plan view of the top shell;

FIG. 15 is a perspective view of the bottom shell and, moreparticularly, of the exposed bottom surface of the bottom shell;

FIG. 16 is a plan view of the interior of the bottom shell;

FIG. 17 is a sectional view of the bottom shell taken along line 17-17of FIG. 16;

FIG. 18 is a plan view of the cutting disc;

FIG. 19 is a cross sectional view of one of the scallops of the cuttingdisc;

FIG. 20 is a perspective view of the impingement plate;

FIG. 21 is a cross sectional view of the impingement plate;

FIG. 22 is perspective view of the plunger;

FIG. 23 is a side view of the plunger;

FIG. 24 is a perspective view of the catch tray;

FIG. 25 is a plan view directed towards the interior of the catch tray;and

FIG. 26 is a side view of the catch tray.

DETAILED DESCRIPTION I. Overview

As seen by reference to FIG. 1 a bone mill 30 of this invention includesa base 32 to which a mill head 34 is removably attached. Internal to thebase 32 is a motor 36 (FIG. 2). A generally planner shaped cutting disc38 (FIG. 18) is disposed inside the mill head 34. A plunger 40 ismounted to the top of the mill head 34. Disposed below the plunger 40,below the underside of the cutting disc 38 is removable catch tray 42.Bone mill 30 is used by actuating the motor 36 so that the motor rotatesdisc 38. Plunger 40 is used to press bone against the rotating disc 38so as to result in the formation of bone chips. These chips fall intocatch tray 42 so that the chips are available for use in a surgicalprocedure.

II. Base

The base 30, as seen in FIGS. 1 and 2, includes a circular foot 46. Aleg 48, having a circular cross section, extends upwardly from foot 46.In the illustrated version of the invention, the top of the foot 46 hasa frusto-conical, inwardly tapered profile such that leg 48 extendsupwardly from the top center of the foot. The leg 48 has a diameter lessthan the diameter of the foot 46.

Leg 48 is formed of metal such as aluminum, stainless steel or plastic.In the illustrated version of the invention, while the leg 48, seen bestin FIG. 3, has a circular cross sectional profile along the whole lengthof the leg, the diameter of the leg is not constant. Specifically, thediameter of the leg 48 tapers inwardly for the first 25% of the alongthe leg from the top of the foot 46. Above this point, the diameter ofthe of the leg 48 tapers outwardly along the remaining length of theleg. In the illustrated version of the invention, leg 48 has a slightlysmaller diameter at the top than at the bottom. A bore 47 extendsaxially through leg 48. Bore 47 has a number of contiguous sections eachwith a diameter different than that of the adjacent section(s). At thetop end of the leg 48, the leg is formed to have an annular inwardlydirected lip 49. Lip 49 defines the top end opening into bore 47.

A pedestal 50 is disposed on top of the leg 48. The pedestal 50, bestseen in FIGS. 4 and 5, has a generally circular cross sectional profile.In the illustrated version of the invention, pedestal 50 has a bottomsurface 52 that tapers outwardly from the top of the leg 48. An arcuateside wall 54 extends upwardly the top of the bottom surface 52. Base 32of the illustrated version of the invention is formed so that thepedestal side wall 54 defines a circle that has a diameter less thanthat of maximum diameter of the foot 46 and greater than the widestdiameter section of the leg 48.

The upper portion of the pedestal side wall 54 is part of an arcuate lip56 that extends around the perimeter of the pedestal 50 and thatprojects above the surrounding top surface 58 of the pedestal. Lip 56and top surface 58 thus define above the top of the pedestal 50 a recess60. Recess 60 is the void space in which mill head 34 seats when mountedto base 32.

Pedestal 50 is also shaped to define a through opening 64 that iscentered on the longitudinal axis of the pedestal. In the illustratedversion of the invention, integrally formed with pedestal 50 is anannular skirt 66 that extends downwardly from the pedestal bottomsurface. The inner surface of skirt 66 defines opening 64. When base 30is assembled, the pedestal skirt 66 is disposed over an adjacentinwardly stepped surface 68 of leg 48 (FIG. 3). Pedestal skirt 66 ispress fit to the top of the leg.

The pedestal 50 is further formed to define a notch 70. Notch 70projects outwardly from opening 64 to the outer perimeter of thepedestal 50. Notch 70 is defined, in part, by two parallel inner sidesurfaces 72 in the body of the parallel (one surface 72 identified inFIG. 5). Inward of lip 56, each inner side surface 72 terminates at anouter side surface 74 that angles outwardly away from the associatedinner side surface 74. Outer side surfaces 74 thus define the outerperimeter of notch 70. The outward flare of the opposed outer sidesurfaces 74 provide notch 70 with a shape, adjacent the outer perimeterof the pedestal 50 that is inwardly tapered. Inward of the outer sidesurfaces 74, parallel inner side surfaces 72 provide notch 70 with arectangular shape.

Pedestal 50 is further formed to have a number of arcuately spaced apartteeth 80 that are disposed around and extend upwardly from the outerperimeter of the pedestal top surface 58. Each tooth 80 is disposedagainst the inner arcuate surface of lip 54. In some versions of theinvention, teeth 80 are integrally formed with the inner arcuate surfaceof lip 54. Each tooth 80 is shaped to have a top, a crown, with a topsurface 82 from which two lateral surfaces 84 extend diagonallydownwardly. In the illustrated version of the invention semi-circulargrooves are formed in the lip 54 on either side of each tooth 80(grooves not identified). These grooves are present for manufacturingpurposes and are otherwise not material to this invention.

A pair of retention arms 88 that are pivotally mounted to the pedestal50 releasably hold the mill head 34 to the base 32. Each retention arm88, as seen in detail in FIG. 6, includes a solid base 90 with agenerally polygonal shape. While the corners of the base 90 aregenerally angular, the top inner corner surface 91 of the base 90, thecorner directed to the top and center of the pedestal, is rounded.Inward of surface 91, arm base is formed with a bore 92. A ledge 93extends inward from the inner side of the arm base 90. The ledge 93 hasa height less than that of the base 90. A lever 96 extends downwardlyfrom the bottom surface of the arm ledge 93. Each arm 88 is furtherformed to have a finger 98 that extends upwardly from the outer topsurface of the base 90. The finger 98 is hook shaped such that at thetop of the finger there is an inwardly directed tab 102 that extend ashort distance of the top surface of the arm base 88.

Each retention arm 88 is operated in a cut-out space 104 defined in thepedestal 50. Each cut-out space 104 is formed, in part, by breaks in thepedestal bottom surface 52 and sidewall 54 shaped to define separatecontiguous void spaces in which the arm base 90 and ledge 93 are seated.Each cut-out space 104 is further formed in part by a break in the outerperimeter of the pedestal lip 56 in which arm finger 98 is seated.Integral with the cut-out spaces 104 are notches in the top of the lip54 in which the finger tabs 102 seat (notches not identified). Retentionarms tabs 102 project into the top of pedestal recess 60.

A pivot pin 108, one seen in FIG. 2, rotatably holds each retention arm88 to the base pedestal 50. Each pin 108 extends through the bore 92 inthe inner upper corner of the associated arm base 90. The opposed endsof the pin 108 are disposed in bore 109 formed in the pedestal 50. (Theopening into one of the pin bores 109 is seen in FIG. 5.) Each pin bore109, it should be appreciated is intersected by the cut-out space 104.

Coil springs 112 (FIG. 2) mounted to the pedestal 50 normally hold eachretention arm 88 in the locked state. Each coil spring 112 has a firstend disposed in a closed-ended, downwardly opening bore 114 formed inthe pedestal 50. Each bore 114 opens into the cut-out space 104 for theassociated arm 50. The opposed end of each spring 114 seats on anupwardly directed surface of the ledge 93 of the associated arm 88.Thus, each spring 112 exerts a force on the associated arm thatpositions the arm so that the tab 102 normally seats in the top locatedpedestal recess 60. As a result of the biasing of the arm by spring 112,it will further be appreciated that each arm level 96 is normallydownwardly directed so as to be generally aligned with base leg 48.

The base motor 36, now described by reference to FIGS. 2 and 4, isdisposed in leg bore 47. In one version of the invention, the motor 36is the motor employed in the Applicant's Assignee's ES6 SurgicalSagittal Saw. This motor is a four-pole, three-phase motor. Thisparticular motor is capable of a no-load maximum shaft speed of at least20,000 RPM. The motor 36 is disposed in the leg 48 so as to be suspendedin the leg 48 above foot 46. Motor 36 has an output shaft (notillustrated) to which a gear head 122 is attached. Extending downwardlyfrom the motor 36 is a flex circuit 123. Flex circuit 123 carriers theconductors over which the motor windings are selectively tied to groundand a voltage source. The flex circuit 123 also supports the conductorsover which the signals from the Hall sensors internal to the motor areoutput, over which a 5VD is supplied to the motor (the Halls) and aground connection is obtained.

The outer perimeter of motor 36 rests on a ring 126 screw secured insidethe leg 48. The ring is threaded into a section 127 of leg bore 47 (FIG.3). Ring 126 is formed with slots 128 located around the outer perimeterof the ring. When base 32 is assembled, the flex circuit 123 extendingfrom the motor 36 is disposed within ring 126.

A gear train 134 is disposed in hollow of leg 48 so as to be disposedover motor 36. The gear train 134, best seen in FIGS. 7 and 8, includesa set of planetary gear assemblies that reduce the speed/increase thetorque of the rotational moment produced by motor 36. Specifically, geartrain 134 includes a tube like housing 136, best seen in FIG. 7A, thatfunctions as the ring gear for the individual planetary gear assemblies.Housing 136 has a smooth outer wall and is designed to closely slip fitinto the hollow bore of leg 48. The gear train housing 136 is formed sothat the lower inner circumferential wall has arcuately spaced apartteeth 138. Above teeth 138 housing 136 is formed to have an innerdiameter that is greater than the inner diameter of the void spaceformed by the toothed portion of the housing. Immediately below the topof the housing 138, the housing is formed to have a groove 140 thatextends circumferentially around the inner wall of the housing. Geartrain housing 136 is further formed so as to have a semi-circularchannel 142 that extends downwardly from the top of the housing alongthe outer surface. Channel 142 extends longitudinally along the housingfor distance that is approximately 15% of the overall length of thehousing.

The bottom of the gear train housing 136 is formed with twodiametrically opposed notches 143. Notches 143 are provided to receive atool used to facilitate the insertion/removal of the gear train 134 fromleg bore 47. Further, when the base 32 is assembled tabs integral withthe motor (tabs not illustrated) seat in the notches 143. The engagementof these tabs with the gear train housing 136 blocks rotation of themotor 36.

Disposed within the gear train housing 136 are three planetary gearassemblies. The first planetary gear assembly includes a carrier disk146. Three gears 148 (two shown in FIG. 7) are rotatably mounted to theunderside of the carrier disk 146, the side directed to motor 36. Gears148 are equangularly spaced apart from each other. Gears 148, like thegears 154 and 182 of, respectively, the second and third planetary gearassemblies are mounted to the carrier disc 146 so that the teeth of thegears 148 will engage the gear housing teeth 138. When base 32 isassembled, gears 148 also engage the gear head 122 of motor 36. A centergear 150 is fixedly mounted to the top side of carrier disk 146. Centergear 150 is coaxial with disk 146.

A second carrier disk, disk 152, forms part of the second planetary gearassembly. Rotatably mounted to the underside of disk 152 are threeequangularly spaced apart gears 154. Gears 154 are sized and positionedto engage both center gear 150 and gear train housing teeth 138. Atubular stem 156 extends upwardly from the top surface of carrier disk152 and is coaxial with the disk 152. Stem 156 is formed to define agear 158 that is located above the surface of center disk 152.

The third planetary gear assembly includes a multi-section drive coupler162 described best by reference to FIG. 9. Drive coupler 162 is shapedto have a cylindrical base 164. At the bottom end of the base a smallannular step 166 projects radially outwardly from the perimeter of thebase of the base. A short distance from the top of the base 164, drivecoupler 162 is formed to have an annular groove 167. Groove 167 extendscircumferentially around the outer surface of the base 164. Formedintegrally with and extending above the base 164, drive coupler 164 isshaped to have a stem 168. Stem 168, while concentric with base 164 hasa diameter less than that of the base.

Drive coupler 162 is further formed to have three coaxial contiguousbores that form a through opening, end-to end, centered along thelongitudinal axis of the coupler. A first bore, bore 169 in FIG. 9,extends from the bottom of the coupler through the longitudinal slicesubtended by step 166 and a section of the base 164 above the step.Immediately above the open end of bore 169, the drive coupler 164 isformed to define an annular groove 170 in the interior wall that definesbore 169. Contiguous with and located above bore 169, the drive coupler162 is formed to define a second bore, bore 172. Bore 172 extendsupwardly from bore 169 to just below the top of the coupler base 164.Bore 172 has a diameter slightly less than that of bore 169. Both bores169 and 172 are greater in diameter than stem 156 integral with thesecond planetary gear assembly. Above bore 172, drive coupler 162 isformed to have a third bore, bore 176. Bore 176 extends from bore 172,through the top of the coupler base 164 and the whole of the stem 168.The open end of bore 176 thus forms the top end opening into drivecoupler. Bore 176 has a diameter greater than that of bore 176.

The drive coupler 162 is further formed to have two diametricallyopposed oval openings 178. Openings 178 are formed in the stem 168 abovewhere the stem emerges from the base 164. Openings 178 have longitudinalaxes parallel to the longitudinal axis of the drive coupler 162. Theopenings 178 thus are contiguous with bore 176.

The third planetary gear assembly has four planet gears 182, (three seenin FIG. 7). Planet gears 182 are mounted to the undersurface of drivecoupler base 164. Planet gears 182 are sized and position to engage 158of the second planetary gear assembly and housing teeth 138.

When gear train 134 is assembled, the second planet gear assembly ispositioned so that stem 156 is disposed with drive coupler bores 169 and172. A bearing assembly 186 provides a low friction coupling between thestem 156 and the drive coupler 162. The outer race of bearing assembly186 is seated in the outer perimeter of the drive coupler bore 169. Atleast one snap ring 185 disposed in drive coupler groove 170 holds thebearing assembly 186 in bore 169. The inner race of the bearing assembly186 is disposed about the outer surface of stem 156 above gear 158. Asleeve 187 formed from stainless steel is press fit over stem 156 abovebearing assembly 186. Sleeve 187 has a lip, not identified that abutsthe upper exposed inner race of the bearing assembly 187. Sleeve 187thus blocks the removal of carrier disc 152 from the drive coupler 162.

Two bearing assemblies 188 and 192 rotatably hold the drive coupler 164to the gear train housing 136. The outer races of both bearingassemblies 188 and 190 are disposed against the smooth inner wall of thegear train housing 136; the inner races are disposed against the outercircumferential wall of the drive coupler base 164. A first one of thebearing assemblies, assembly 188, is positioned so that the outer raceof the assembly seats on the step defined by tops of the housing teeth138. The inner race of bearing assembly 188 is seated against the drivecoupler step 166.

A ring shaped spacer 190 separates bearing assemblies 188 and 192.Spacer 190 is disposed against the smooth inner wall of the gear trainhousing. Spacer is sandwiched between the outer races of both bearingassemblies 188 and 192.

A retaining ring 196 is snap fitted in groove 140 internal to the geartrain housing 134. Retaining ring 196 extends over the outer race of thebearing assembly 192. Retaining ring 196 thus holds bearing assembly 192and the components disposed below the assembly 192 in the gear trainhousing 136.

A spindle 202, now described by initial reference to FIG. 10, extendsfrom the drive coupler 162 and transfers the torque developed by themotor 36 to the mill cutting disc 38. Spindle 202 is shaped to include agenerally solid post 204. Post 204 has a diameter that allows it toslidably fit in bore 176 internal to the drive coupler stem 168. Post204 is formed to have a laterally extending through bore 206.

Above the post 204, spindle 202 is shaped to have a disc-shaped head208. The spindle 202 is formed so that head 208 has a diameterapproximately equal to that of the drive coupler base 164. A number ofdifferent components extend upwardly from the top surface of the spindlehead 208. One of these components is an alignment pin 210. The alignmentpin 210 is coaxial with the longitudinal axis of the spindle 202; thepin 210 extends upwardly from the center of the head. Pin 210 is shapedso that the lower portion, the portion that extends upwardly from thespindle head 208, has a cylindrical shape. The top portion of alignmentpin has a shape of a cone with a flattened tip. (The individual sectionsof alignment pin 210 are not identified.)

Four equangularly spaced apart alignment teeth 212 also extend upwardlyfrom the top surface of the spindle head 208. The teeth 212 are locatedaround the outer perimeter of the spindle head 208. The arcuate outersurfaces of teeth 212 are flush with the outer surface of the spindlehead 208. Each tooth has a pair of inwardly tapered side surfaces and anarcuate inner surface. (Surfaces not identified.) The radius ofcurvature of the inner surfaces of each tooth 212 is less than theradius of curvature of the outer surface. Teeth 212 do not extend as farabove the spindle head 208 as does alignment pin 210.

Spindle 202 is positioned so that post 204 is slidably mounted in thedrive coupler stem bore 176. A pin 214, extends through spindle bore206. The opposed ends of the pin 214 are seated in the oval openings 178formed in drive coupler stem 168. Pin 214 thus holds the spindle 202 tothe drive coupler 162 so that the spindle rotates in unison with thedrive coupler and is able to move longitudinally relative to the drivecoupler.

A spring 216 is disposed in the drive coupler 162 below spindle 202.Spring 216 is a wave spring. One end of spring 216 is seated on theannular step internal to the drive coupler between bores 172 and 176.The opposed end of spring 216 is disposed against the bottom end ofspindle post 204. Spring 216 is selected so to exert an upward bias onspindle post 204. Thus force, which can be overcome by the applicationof manual force, serves to normally urge the spindle head 208 away fromthe drive coupler 162.

When gear train 134 is mounted in leg 48, an anti-rotation pin 218 isseated in the channel 142 formed in gear train housing 136. The exposedportion of pin 218 seat in a complementary groove formed in the interiorwall of the leg 48 that defines the section of leg bore 47 in which thegear train is seated (groove not illustrated). Anti-rotation pin 218thus inhibits the rotational movement of the gear train housing 136.

In some versions of the invention, the motor 36 and gear train 134 arecollectively provided so that the spindle 202, and by extension, thecutting disc 38 are able to rotate at speeds of between 150 and 500 RPM.In some versions of the invention, the components internal to the base32 are selected so that the disc can be rotated at speeds between 250and 350 RPM. These speeds are understood to be the under-load speeds ofthe cutting disc 38, when bone stock is pressed against the disc.

Returning to FIG. 2 it can be seen that a spring biased, normally openpress button-switch 220 is mounted to the outer top surface of the basefoot 46. Switch 220 is mounted to a plate 222. Plate 222 is disposed ina recess 224 in the outer surface of the foot 220. A socket 226 isdisposed in an opening in the outer circumferential wall of foot 46.Socket 226 receives the below described that connects the base to thebelow described control console that actuates bone mill 30. Notillustrated are the circuit board and conductors internal to the base 32over which conductive connections from the motor flex circuit 122 andswitch 220 to the pins internal to the socket 226.

Seen in FIGS. 2 and 4 is the base plate 228 that is disposed over theopen bottom end of base foot 46. Not illustrated are the fasteners thathold plate 228 to the foot 46.

III. Mill Head

The mill head 34, as seen by FIG. 11, includes top and bottom shells 240and 242, respectively. When assembled together, shells 240 and 242 formthe housing of the mill head 34. Cutting disc 38 is disposed between theshells. Mounted to the top shell 240 is an impingement plate 244. Theplunger 40 is slidably mounted to the top shell 240 so the bottomsurface thereof is directed towards the cutting disc 38. Catch tray 42is mounted to two parallel rails 358 and 360 (FIG. 15) integral with thebottom shell 242. The catch tray 42 is mounted to the mill head 34 so asto be located below the plunger 40. Owing to the mounting of the catchtray to rails, the tray can be slide radially away from and removed fromthe rest of the mill head 34.

Referring to FIGS. 12, 13 and 14, it can be seen that mill head topshell 240 is formed from a single piece of plastic. One suitable plasticfrom which top shell is a polycarbonate plastic MAKROLON RX2530available from Bayer Material Science AG of Leverkusen, Germany. Itshould be appreciated that the material from which the mill head topshell 240 as well as the other components forming the top shell isformed from, is material that can be successfully subjected to gammasterilization.

Top shell 240 is shaped to have a circular head 250 that is generallydisc shaped. The bottom surface 251 of head 250 can generally bedescribed as planar. The top shell 240 is further formed so that the topsurface of the head immediately inward of the perimeter of the head,surface 252, is parallel with the bottom surface of the inward. Inwardof this perimeter surface 252, top shell head 250 is formed with acenter surface 254 that has a slight rise such that the center of top islocated above the outer perimeter surface 252. Top shell head 250 isfurther formed as to have diametrically imposed indentations 256 each ofwhich intersects perimeter surface 252. Indentations 256 arediametrically opposed from each other. Top shell head 250 is furtherformed so that each indentation has a side-to-side length that isslightly greater than the width of a base retention arm 88.

An arcuate outer lip 255 extends downwardly from the bottom surface ofthe top shell head 250. Lip 255 is L-shaped such that inward side of thelip there is a small step 256 that is parallel with the bottom surfaceof the top shell head 250. Head 250 is further formed so that spacedinwardly from outer lip 255, and concentric with the outer lip 255 thereis an inner lip 260. Inner lip 260 has a rectangularly shaped crosssectional profile. Specifically, inner lip 260 extends downwardly sothat the bottom surface of lip 260 is coplanar with step 256 integralwith the outer lip 255. Between the outer and inner lips 254 and 260,respectively the head 250 defines an annular groove 258. Groove 258 hasa base that relative to bottom surface 251 is recessed.

Disposed inwardly of the inner lip 255, top shell head 242 is formed tohave a circular outer containment ring 264. Containment ring 264projects downwardly from the head 250 and has a rectangular crosssectional profile. Between the inner lip 260 and the outer containmentring 264 there is an annular groove 262. The base of groove 262 is inthe same plane as that of the surrounding groove 258.

Lips 254 and 260 while curved, are not unbroken circles. Below thesections of outer perimeter surface in which indentations 252 areformed, top shell 232 is molded so that two diametrically opposedrectangular blocks 268 intersect grooves 258 and 262 and inner lip 260.Top shell 240 is further formed so that four equangularly spaced apartgaps 272 separate the outer lip 255 into four sections. Radially alignedwith each gap 272 is a slot 274. Each slot 274 separates sections of theinner lip 260 and has a base that extends further into the shell head250 deeper than the bases of the sections of the grooves 258 and 262intersected by the slot 274. Each slot 274 subtends an arc greater thanthat subtended by the adjacent and contiguous outer lip gap 272.

The top shell head 250 is further formed so as to have circular innercontainment ring 276 located outwardly of the center of the head. Innercontainment ring 276 has a rectangular cross sectional profile. FromFIG. 14 it can be seen that the outer and inner containment rings 264and 276, respectively, define the outer and inner perimeters of the headbottom surface 251. Head 250 is further shaped so that the containmentrings 264 and 276 extend downwardly the same distance from bottomsurface 251. Also, the surface of the head 250 within the innercontainment ring 276 is further from the exposed face of the ring 276than bottom surface 251. Top shell head 250 is also shaped to define adome shaped void space 275 that is concentric with the head.

Top shell 240 is further shaped so that there is an opening 278 in head250. Opening 278 extends from an arcuate edge in the head immediatelyoutward of inner containment ring 276 to an arcuate edge immediatelyinward of the outer containment ring 264. These arcuate edges areconcentric with the center axis of the head 250. Opening 278 is furtherdefined by two parallel side edges in the head 250. Each of the sideedges extends between opposed terminal ends of the associated arcuateinner and outer edges.

The top shell head 250 is further formed to define a void space 280 thatextends inwardly from bottom surface 251 adjacent opening 278. Voidspace 280 is formed by an inward step in the material forming the head250 (step not identified). Void space 280 is positioned to be contiguouswith one side of opening 278. More particularly, the void space isadjacent the side of opening 278 towards which the cutting disc 38rotates when bone mill 30 is actuated. The top shell is further formedto define two mounting posts 282 that extend from head into void space280. Given the inversion of FIG. 14 relative to the actual orientationof the top disc, it should be understood that posts 282 projectdownwardly from surface defining the top of the space 280 into thespace.

The top shell head 250 also includes three angularly spaced apartreinforcing ribs 284, 286 and 288. Each of the ribs 284-286 extendsradially from the inner containment ring 276 to the outer containmentring 264. Each rib 284-286, also projects downwardly from the headbottom surface 251 a distance less than that which the containment rings264 and 276 downwardly extend. Two of the ribs, ribs 284 and 286, arelongitudinally centered over radial lines that project from the centerof the head 250. The third rib, rib 288, has a number of differentsections. There is an inner section that projects from the innercircumferential ring 276 around the inner edge of void space 280. Amiddle section extends around the forward edge of void space 280, theedge spaced from opening 278. The third section of rib 288 extends fromthe middle section to the outer containment ring 264. This third sectionis centered on a radial line that projects from the center of the head250. The radial lines along which ribs 284 and 286 and the outersection, the third section of rib 288 are centered are equangularlyspaced apart from each other.

A hollow feed sleeve 292 also part of top shell 240 extends above head250 and is disposed around opening 278. In the illustrated versions ofthe invention, sleeve 292 is a four-walled structure. There is an innerwall 294 adjacent the center of the head 250. Inner wall 294 has threesections. There is a center section that is inwardly curved and twocoplanar outer sections (individual sections not identified). There isan outer wall 298 has an arcuate profile. Sleeve 292 has two sideparallel side walls 296. Each side wall 296 extends between the outeredge of one of the outer sections of the inner wall 294 and the adjacentouter edge of the outer wall 298. Not identified is the hollow internalto sleeve 292 defined by walls 294-298. This hollow opens into headopening 278. More particularly, top shell head 250 is formed so thatopening 278 has a cross sectional profile identical to that defined bythe hollow of sleeve 292.

Top shell 240 is also formed with reinforcing members adjacent sleeve292. In particular, a web 304 flares out from and is coplanar with eachside wall. Each web 304 extends over the head center surface 252 on theside of the head opposite the side from which the sleeve 292 projects.

Bottom shell 242 of the mill head housing, as seen in FIGS. 15-17, isshaped to have a generally circular shaped body 308. The bottom shell242 can be formed from the same material from which the top shell 240 isformed. While approximating the shape of disc, shell body 308 is furtherformed to have through hole 310. Hole 310 is concentric with thelongitudinal axis of body 308.

The shell body 308 is further formed to define an opening 312 separatefrom hole 310. Opening 312 extends inward from body 308 from an arcuateedge in the body located inward of the outer perimeter of the body.Opening 312 is further defined by two parallel interior walls thatextend inwardly from the arcuate outer edge. One of the side walls, wall314 in FIG. 15, is outwardly beveled such that it extending diagonallyfrom the top of opening 312 outwardly to the exposed bottom surface ofthe shell body 308. Wall 314 also extends upwardly a short distance fromthe upper inner surface of shell body 308. The side wall opposite wall314, wall 316 (FIG. 16) extends perpendicular to the planar top face ofthe shell body 308. The shell body 308 is formed so that walls 314 and316 extend on either side of the portion of the body that defines holes310. An inner wall 318 extends between the side walls 314 and 316 todefine the inner perimeter of opening 312. Inner wall 318 has a numberof different sections, (not identified.) Adjacent each side wall 314 and316, the inner wall has an outer section that extends perpendicularlyinwardly from the adjacent side wall 314 or 316. The inner wall outersection that abuts side wall 314, like side wall 314, is inwardlytapered. Between there outer sections, inner wall 318 has an inwardlycurved middle section. The portion of the shell body 308 that definesthe middle section of inner wall 318 also defines an adjacent arcuatesection of hole 310.

A multi-section lip 320 extends upwardly from the outer perimeter ofshell body 308. Lip 320 is formed to define first and second outer steps322 and 324, respectively. The first outer step 322 is the step locatedfurthest from the center of the shell. Second outer step 324 is locatedimmediately inward of the first outer step 322. Step 324 is locatedabove the inner step. Lip 320 is further formed to define a crown 326located inwardly of and that extends above the second outer step 324.The crown 326 is formed to have a pyramid-shaped cross-sectional profileso as to define a peak 330. Crown 326 also has outer and inner parallelside surfaces 328 and 332, located on the opposed sides of the peak 330.The side surfaces 328 and 332 thus project upwardly and areperpendicular to the plane of outer step 324. Shell lip 320 is alsoshaped to have an inner step 334. Inner step 334 is coplanar with thesecond outer step 324.

Top and bottom shells 240 and 242, respectively, are further formed sothat, when assembled, bottom shell crown 326 tightly fits in top shellgroove 258. Also the bottom shell first outer step 322 extends over andhas an outer diameter generally equal to the most overlying outer mostbottom located arcuate surface of top shell outer lip 255. Further theshells 240 and 242 are mutually shaped so that, when assembled together,bottom shell opening 312 subtends the area subtended by top shellopening 278 and the adjacent void space 280.

Four equangularly spaced apart notches 340 are formed in the bottomshell 242. The bottom shell 242 is further formed so that, immediatelyinward from each notch 340 there is a downwardly extending finger 342.Each finger 342 is shaped to have a curved inner and outer surface,(surfaces not identified). The side-to-side width of each finger 342 issuch that the finger can seat in one of the top shell slots 274. Eachfinger 342 has an inner-to-outer surface depth such that the fingersubtends an arcuate section of the bottom shell that would otherwise besubtended by a section of the lip inner step 334 and the adjacent innersurface of the shell body 308. A reinforcing tab 344 is formed integralwith each finger 342 so as to extend over the inner surface of thefinger. Each tab 344, like the associated finger 342, extends upwardlyform the adjacent top located surface of the shell body.

Two of the sections of lip 320 are formed with their own diametricallyopposed breaks 348. Each break 348 essentially separates the crown 326of the associated lip section into two sections. Each break 348 definesan arcuate void space dimensioned so that, when the mill head 34 isassembled, one of the top shell blocks 268 seats in the break.

Located immediately of the circle defined by reinforcing tabs 344, thebottom shell 242 is formed to define a circular outer containment ring350. Containment ring 350 extends upwardly from the inner surface of theshell body and has a rectangular cross-sectional profile. Bottom shell242 is further shaped to have an inner containment ring 352. Innercontainment ring 352, extends upwardly from the inner surface of theshell body 308 and has an inner diameter slightly greater than that ofcenter hole 310. Like the outer containment ring 350, the innercontainment ring has a rectangularly shaped cross-sectional profile.Inner containment ring 352, it should be noted extends over the arcuatesection of the shell body 308 that separates hole 310 and opening 312.The outer diameter of the inner containment ring 352 is slightly lessthan the arc that defines the middle section of the opening 312 defininginner wall 318. In the illustrated version of the invention, formanufacturing reasons, adjacent where the containment ring and side wall318 the ring is not circular.

Shells 240 and 242 are further constructed so that, when assembledtogether to form the mill head housing, the top shell containment rings264 and 276 overlap the bottom shell containment rings 350 and 352,respectively. When the mill head 34 is assembled, shells 240 and 242 areultrasonically welded together.

A reinforcing rib 354, best seen in FIG. 16, projects away from theinner containment ring 352 of the bottom shell 242. Rib 354 extends overthe inner surface of the shell base 308. The rib 354 extends from ring352 over the portion of the body 308 that defines the section of theinner wall 318 adjacent side wall 314. Owing to its rectangularcross-sectional profile 354, rib 354 is actually flush with the adjacentsection of inner wall 318. At the end of inner wall 318, rib bendsperpendicularly to extend over the surface of the shell body adjacentthe top edge of side wall 314. While rib 354 extends toward the outerperimeter of the shell base the rib terminates a short distance inwardof the outer containment ring 350.

Two parallel rails 358 and 360 project downward from the underside ofshell base 308. Rails 358 and 360 are the structural members integralwith the mill head housing that slidably hold the catch tray 42 to therest of the mill 34. The rails 358 and 360 are both parallel to opening312—defining side walls 314 and 316. Rail 358 extends downwardly fromthe housing body 308 adjacent side wall 314 so as to be spaced away fromopening 312. Rail 358 is in the form of a rectangular structure thatextends diagonally towards opening 312. Rail 360 extends from theundersurface of the shell body 308 adjacent side wall 316 so as to bespaced from opening 312. Rail 360 is generally in the form of arectangular structure that extends diagonally toward opening 312. Therails are different in that rail 360 is longer than rail 358. Also,teeth, (not identified) extend from the main body of rail 360. The teethare present for manufacturing purposes and are otherwise not relevant tothis invention. Similar teeth may also project from shell body side wall316. Again, these teeth are present for manufacturing purposes.

A pair of tabs 364, also project downwardly from the undersurface ofshell body 308. Tabs 364 are located between the outer perimeter or thebottom shell 242 and rails 358 and 360. A first one of the tabs 364 isadjacent rail 358 and is oriented to extend away from the longitudinalaxis along which rail 358 is centered, away from opening 312. The secondtab 364 is adjacent rail 360 and is oriented to extend outwardlyrelative to opening 312. Tabs 364 are generally in the shape ofrectangular blocks. The tabs 364 thus serve as alignment members thatfacilitate the centering of the catch tray 42 between rails 358 and 360.

Cutting disc 38, now described with initial reference to FIG. 18, isformed from a material that can be appropriately shaped, that will notfatigue when used to cut bone as described below and that can besubjected to sterilization processes. In some versions of the inventionthe cutting disc 38 is formed from stainless steel such as 410 StainlessSteel. Generally, the disc 38 has a circular shape with opposed top andbottom surfaces 370 and 372, respectively. The diameter of the cuttingdisc is approximately 0.10 cm greater than the outer diameter of theouter containment rings 264 and 350 integral with the mill head housing.

The cutting disc 38 is further shaped to have a center-located hole 374.Hole 374 is dimensioned to receive the alignment pin 210 integral withthe base spindle 202. Located around hole 374, the cutting disc 38 isformed to have four equangularly shaped apart openings 376. Each opening376 is shaped to receive a separate one of the teeth 212 integral withspindle 202. Accordingly, openings 376 are arcuately shaped. The circledefined by the outer circumference of openings 376 is less than innerdiameter of the inner containment rings 276 and 352 integral with themill head housing.

Cutting disc 38 is further formed to have a number of cutting scallops378. Integral with and longitudinally axially aligned with each cuttingscallop 378, the cutting disc has a through opening 380. As seen in FIG.19, each scallop 378 is formed by shaping the cutting disc 38 so thatthe disc top and bottom surfaces 370 and 372, respectively, adjacent theopening 380 curve upwardly into, respectively, a scallop top surface 382and a scallop bottom surface 386. The scallop top surface 380 is milledso that top surface 382 and bottom surface 386 meet at an edge 384. Thisedge 384 is the cutting edge of the scallop 378.

Edge 384 is also the edge of the scallop that defines the perimeter ofthe associated opening 380. Each opening 380 is generally rectangularlyshape wherein the long sides extend forward from the scallop 378 withwhich the opening is integral. Each opening 380 is not exactly in theshape of a rectangular in that the sides of the opening, including theside defined by edge 384 are outwardly bowed. Further, the corners wherethe sides of the opening 380 meet are rounded.

The cutting disc 38 is formed so that the scallop 378-opening 380 pairsare arcuately and radially spaced apart from each other around the disc.The scallop 378-openings 380 located closest to the center of the discare radially spaced from the center to lie outside a circle on the dischaving a diameter equal to the outer diameter of the inner containmentrings 276 and 352. The outermost located scallop 378-opening 380 pairsare located within the circle on the disc that has diameter equal to theinner diameter of the outer containment rings 264 and 350. The scallop378-opening 380 pairs are further arranged so that no two scallop edges384 are on the same radial line projecting from the center of the disc38.

When the mill head 34 is assembled, the cutting disc is sandwichedbetween the downwardly-directed, top-located containment rings 264, 276and the upwardly-directed, bottom-located containment rings 350 and 352.Owing to the relative dimensioning of the components, the cutting disc38 has a top surface 370 to bottom surface 372 thickness that isapproximately 0.009 inches (0.23 mm) less than the gap between thealigned containment ring pairs 264-350 and 276-352. Consequently, thecutting disc 38 is able to slightly move, float, in three degrees offreedom within the mill head housing.

The impingement plate 244, shown in FIGS. 20 and 21, is formed frommaterial that, in addition to being sterilizable, will not fracture whenbone is pressed against it. In some versions of the invention, theimpingement plate is formed from stainless steel such as 304 stainlesssteel. The impingement plate 244 is generally in the form of a blockthat has opposed top and bottom surfaces 388 and 389, respectively. Theplate is dimensioned to seat in void space 280 defined within top shell240. Accordingly, one of the side walls of the impingement plate iscurved, (curved side not identified.) Impingement plate has a front face391 that, in terms of the length is the longer of the two faces of theplate. (In FIG. 20, the edge of front surface 391 is identified.)Impingement plate 244 is further formed so that around the top surface388 of the plate there is downwardly extending bevel 392.

Two bores 394 extend through the impingement plate from the top surfaceto the bottom surface. Each bore 394 actually opens from a largerdiameter counterbore 395 that extends upwardly from the plate bottomsurface 389.

When mill head 34 is assembled, impingement plate 244 seats in top shellvoid space 280. The impingement plate is positioned so that the frontface 391 serves as the surface defining the front of opening 278. Platetop surface 388 abuts the interior of shell 240 that defines the roof ofvoid space 280. When the impingement plate is so positioned, each post282 seats in a separate one of the bore 394-counterbore 395 pairs. Theimpingement plate is secured to top shell 240 by heat deforming the tipsof the posts 282. The melted plastic form rivets within the counterbores395.

FIGS. 22 and 23 illustrate plunger 40. The plunger 40 can be formed outof the same material from which the top shell 240 is formed. The plunger40 is formed to have a head 402 from which a rod 404 extends. Rod 404 isshaped to have two parallel side panels 406 and a front panel 408 thatextends between the side panels. A bottom plate 410 extends between theside panels 406 and front panel 408 to form the base or bottom of therod 404. Rod 404 is dimensioned so that the side panels 406, the frontpanel 408 and bottom plate 410 can slidably fit in the housing feedsleeve 250. Accordingly, the bottom panel 410 has an exposed edge withan inwardly curved section 412. The curvature of edge section 412conforms to and is slightly greater than the curvature of the feedsleeve inner wall 294.

Plunger rod 404 also includes a top plate 414. The plunger 40 is formedso that the top plate 414 extends over and projects beyond the side andfront panels 406 and 408, respectively. More specifically, the top plate414 is dimensioned to subtend an area larger than the cross-sectionalarea of the center void of the housing feed sleeve 250. The top plate414 thus limits the extent to which the plunger rod 404 can be pushedinto the sleeve and the opening 278 immediately below the sleeve. Insome versions of the invention, the components of mill head 34 aredimensioned so that when the plunger 40 is completely disposed in sleeve250 the bottom plate 412 is at least 0.05 cm above the cutting disk 38.

The plunger 40 is further shaped so that head 402 extends diagonallyupwardly from the outer perimeter of the rod 404. A number of webs 416extend between the top of top plate 414 and head 402. Webs 416 providereinforcing strength to prevent bending of the head 402 relative to thetop plate 414.

As seen in FIGS. 24-26, catch tray 42 is formed to have opposed frontand rear panels 420 and 422, respectively with side panels 424 locatedtherebetween. A bottom plate 426 extends between the panels at thebottom of the tray 42 to find the base of the tray. In the illustratedversion of the invention, front panel 420 has a arcuate profile. Foraesthetic reasons, at the top of the panel this profile has a radius ofcurvature equal to the diameter of top shell 240.

Tray rear panel 422 has a number of different sections. A bottom section430 extends upwardly from tray bottom plate 426. More particularly, thebottom section extends diagonally upwardly away from plate 426, Panelbottom section 430 is substantially planar. Contiguous with and locatedabove the bottom section 430, rear panel 422 has a top section 432. Thetop section, like the bottom section extends diagonally away from thebottom plate 426. The outward taper of the panel top section 432 isgreater than that of the bottom section 430. Further the catch tray isformed so that there is a circular indentation 436. Indentation 436 hasa curvature slightly greater than that of outer perimeter of the arcuatesection of the bottom shell body 308 that separates hole 310 fromopening 312.

A lip 438 extends upwardly from rear panel top section tray rear paneltop section 432. Lip 438, it will further be observed, extends above thetray side panels 424. Lip 438 has the same curvature of indentation 436.Thus, when the tray 40 is mounted to the mill head housing, the outersurface of the lip can abuts the arcuate section of the bottom shellbase 308 that defines the inner perimeter of opening 312.

Side panels 424 are parallel planar structures. On each side of thecatch tray 40 a separate one of the side panels 424 extends between theopposed side edges of the front panel 420 and the rear panel 424. Aflange 440 extends diagonally outwardly from the top edge of each sidepanel 424. Flanges 440 are dimensioned to seat against the innersurfaces of the bottom shell rails 358 and 360.

IV. Operation

Bone mill assembly 30 of this invention is prepared for use by firstattaching mill head 34 to base 32. This is accomplished by placing thebase so that the bottom shell 242 seats in the pedestal recess 60. Aninitial part of this process is the alignment of the mill head so thatcatch tray 42 is positioned in pedestal notch 70. As a result of thisarrangement of components, the longitudinal axis of the mill head 34 isapproximately aligned with the longitudinal axis of the base 32. As themill head bottom shell 242 starts to seat in the pedestal recess 60, thespindle alignment pin 210 extends through shell hole 310. Moreparticularly, the alignment pin 210 extends through the bottom shellhole 310 and enters the cutting disc center hole 374.

Since the cutting disc floats within the mill head housing, the furtherseating of the mill 34 on the base results in the cutting disc 38centering itself on the drive spindle alignment pin 210. As the millhead 34 is further seated against the base 32, the spindle teeth 212 mayor may not be aligned with, be seated in, the complementary openings 376formed in the disc 36. In either situation, the movement of the cuttingdisc 36 against the spindle 202 overcomes the force spring 216 places onthe spindle to urge the spring upwardly. Thus, the seating of the millhead 34 in the base recess 60 results in at least some retraction of thespindle 202 toward the base foot 46. The maximum retraction occurs ininstances in which the spindle teeth 212 are not seated in the discopenings 376.

As part of the fitting of the mill head 34 on the pedestal, pedestalteeth 80 seat in bottom shell notches 340. As a consequence of thefitting of the mill head catch tray 42 in the pedestal notch 70, thepedestal teeth 80 are inherently aligned with the complementary bonebottom shell notches 340. Further, as a result of this seating of themill head 34 in the pedestal recess 60, each indentation 252 in the millhead top shell 240 is positioned adjacent a separate one of therestraining arms 88.

The mill head 34 is then secured to the base by pivoting the retentionarms 88 upwardly. This displacement of a retention arm results in thetip of the arm finger 98 seating in the adjacent indentation 252 in themill head top shell 240. Spring 112 holds the arm 88 in this position.

As part of the process of preparing the bone mill for use, the base 32is connected to a complementary console that provides signals forenergizing the motor 36. One such console is sold by the StrykerCorporation of Kalamazoo, Mich. as the CORE™ Console. A description ofthe circuitry internal to this console that can be used to power motor36 is presented in U.S. Pat. Pub. No. US 2006/0074405 A1, the contentsof which is incorporated herein by reference. A cable (not illustrated)extends from base socket 226 to a complementary socket integral with theassociated console. Conductors over this cable provide the on/off signalto the console; selectively apply energization signals to the windingsinternal to the motor; and provide the signals from the sensors internalto the motor back to the control console. The above-mentioned COREConsole is capable of actuating surgical instruments other than bonemill 30 of this invention. In other versions of the invention, a consolespecifically designed to energize motor 36 may be provided. Accordingly,the exact structure of the console used to supply the energizationsignals to the motor 36 is not relevant to this invention.

Bone, either allograft or autograft in nature, is loaded in the millhead feed sleeve 292. Once the bone is so loaded, the plunger 40 isdisposed in the feed sleeve.

The bone chips are produced by simultaneously actuating the cutting disc38 while the plunger 40 is depressed to force the bone against the disc.Cutting disc 38 is actuated by depressing switch 220 so as to cause theactuation of the base motor 36. The actuation of the motor results inthe rotation of spindle 202.

As mentioned above, when the mill head is fitted to the base, thespindle teeth 212 may already be engaged in the disc openings 376. Inthis situation, the rotation of spindle 202 results in the immediatelike rotation of the cutting disc 38. There is, however, the potentialthat initially the cutting disc 38 and spindle 202 are not so aligned.In this event, the initial rotation of the spindle results in thespindle teeth 212 engaging in an arcuate path of travel against the discbottom surface 312. Teeth 212 so move against the disc until the teethcome into registration with the disc openings 376. When this eventoccurs, spring 216, which exerts an upward force on the spindle 202,pushes the spindle upward so that the teeth 212 seat in the discopenings 376. Once this event occurs, the cutting disc 38 rotates inunison with the spindle 202.

The rotation of the cutting disc 38 means that the disc scallops 378 arerotated towards the impingement plate 244. More particularly, owing tothe assembly of the mill head 34, the scallops 378 are rotated so asthey pass under top shell opening 278, scallop edges 380 rotate towardsthe impingement plate 244. As mentioned above, simultaneously with therotation of the cutting disc 38, the plunger 40 pushes the basic bonestock through opening 278 against the cutting disc 38. As disc 38rotates, the lower portion of the bone stock is wedged between thescallop edges 384 and the impingement plate front face 391. Thecontinued rotation of the cutting disc causes the disc edges 384 toshear the bone wedged between the disc scallops and the impingementplate away from the bone stock. A sheared bone chip enters the cuttingdisc 380 opening defined by the disc edge that sheared the chip from thestock. From the disc opening 380, the chip falls through the bottomshell opening 312 into catch tray 42.

During the chip formation process, some chips may initially adhere tothe disc bottom surface. These chips abut bottom shell wall 314. Wall314 thus serves as a wiper that prevents the bone chips from rotatingwith the disc 38. The chips, if they adhere, remain in the area abovebottom shell opening 312. Also during the chip formation process, innercontainment ring 352 integral with bottom shell 242 functions as abarrier that prevents the chips from being discharged through opening310. Inner containment ring 276 integral with top shell 240 serves asbarrier that prevents lose material integral with the bone stock frommigrating towards the center of the cutting disc 38 where the materialcould exit through openings 374 and 376.

In the described version of the invention, the reduction in speed of therotational motion of the motor shaft by gear train 134 significantlyincreases the torque at stall the available from the drive spindle 202.Specifically, in some versions of the invention, the gear train reducesthe speed of the spindle down to between 250 and 300 RPM. At a speedwithin this range and using the described motor, the spindle is able tooutput at least 75 inch/pounds of torque and usually 100 inch/pounds ormore of torque. Since the spindle is able to provide this relativelyhigh quantity of torque, the likelihood that, when a piece of bone stockis pressed against the cutting disc 38, motor 36 will stall issubstantially eliminated.

Owing to the relative dimensioning of the mill 34 components, theplunger top plate 414 functions as a stop that prevents the bottom ofthe plunger rod 404 from being pressed against the cutting disc 38.

Once a sufficient volume of bone chips have been formed the motor 36 isstopped. Catch tray 42, with the bone chips contained therein, isremoved from the mill head 34. As a result of the process of sliding thecatch tray 42 away from the mill head housing, tray lip 438 sweeps belowthe section of the disc bottom surface 372 disposed immediately abovebottom shell opening 312. Lip 438 catches bone chips that may beadhering to this section of the cutting disc 38 and the underside of thewiper rib 314. The bone chips are retrieved from the catch tray 42 foruse.

The mill head 34 of this invention is provided with a planar cuttingdisc 38. Owing to disc 38 having this geometry it can be more economicalto provide than other cutting blades such as a cylindrical or bent angleblade. The cost minimization associated with this component helps makeit possible to provide the mill head 34 as a pre-sterilized, useonce-and-dispose component. Thus, between uses of bone mill 30, medicalpersonnel do not have to concern themselves with sterilizing the cuttingdisc 38, with all its sharp edges 384.

As discussed above, bone mill 30 of this invention is furtherconstructed so that the seating of the mill head 34 on the base 32results in the alignment of the floating cutting disc 32 with the drivespindle 202. The need to provide a means to precisely hold the cuttingdisc in a fixed position and so that it can rotate around that positionis eliminated. The elimination of the cost associated with providingthis sub-assembly further contributes to the economics that make itpossible to provide mill head 34 as a use-once disposable.

Cost minimization is further achieved by the fact that holes 376 serveas the features that couple the cutting disc 38 to the drive fasteners.The need to provide supplemental fasteners to perform this function iseliminated.

Bone mill 30 of this invention is further constructed, so that as soonas most chips are cut, they are ejected through the opening 380associated with scallop 378 that cut the bone. From opening 380, thechips typically falls into catch tray 42. Thus, substantially all thebone chips formed in the mill head 34 are subjected to a single cut, asingle pass against either the cutting disc 38 or impingement plate 244.Since the bone chips are only subjected to a single pass against thesecomponents, the amount of frictional heating to which the bone chips areexposed as a result of such contact is likewise held to a minimum. Thisreduction in chip heating results in a like minimization of the extentto which such heating damages the material forming the chips.

Another benefit gained by the design feature that each chip is onlypressed against the cutting disc 38 or impingement block 244 once isthat the chip is not reduced to below the desired size. Further, fewchips, when cut may adhere to the disc bottom surface 372. Substantiallyall the chips, immediately on formation, are forced into the catch tray42. Thus, the chips formed with mil 30 are generally of the same size,

Base 32 and mill head 34 of the assembly 30 of this invention arefurther constructed so that as consequence of the pedestal teeth 70seating in mill indentations 252, the teeth inhibit the transfer of therotational moment of the cutting disc 38 to the rest of the mill 34. Therotational blocking effect of these teeth 70 minimizes the extent towhich the restraining arms 88 have to likewise perform this function.This rotation-stopping effect of the teeth serves to minimize the sideand/or number restraining arms. Thus in most versions of the invention,a maximum of two restraining arms or other fastening members arerequired to releasable hold the mill 34 to the bone 32.

Once a mill head of this invention is used, the catch tray 42 isreattached from the mill head housing. Mill head 34 is disconnected frombase 32 by pivoting the restraining arms 88 away from the mill top shell240. The biasing force of spring 216 displaces the mill a slightdistance above the base pedestal 50. Thus, spring 216 facilitates theseparation from of the mill head 34 from the base 32. Moreover, postuse, the openings 278 and 312 in the mill housing that expose thecutting disc 38 are covered by, respectively, the plunger 40 and thecatch tray 42. This reduces the likelihood that persons handling themill 34 post use, could inadvertently come into contact with thebiological material and sharp disc edges 384 inside the housing. Also,given the presence of the sleeve 296 over opening 278 and the relativelysmall size of opening 312, even if the plunger and catch tray areremoved, it is unlikely fingers can come into contact with the disc 38.

Still another feature of bone mill 30 of this invention is that bychanging the mill head 34, the mill can be used to provide chips ofdifferent sizes. More particularly, individual mill heads can beprovided with cutting discs 38 that have different sized openings. Hereopening “size” is defined by the width across the opening below thecutting edge 384. In one version of the invention, this width may be 8mm across. Mill heads with this size disc openings are used to formlarge sized, coarse sized, bone chips. A mill head 34 with a disc 38having openings 380 with widths of 5 mm across is used to form mediumsized bone chips. Alternatively, one could use a mill head 34 with acutting disc 38 that has openings with a width of 3 mm across. Thisparticular type of mill head is used to form fine sized bone chips.

V. Alternative Embodiments

The above is limited to one specific version of the bone mill of thisinvention. Alternative versions are possible. For example, there is norequirement that all versions of the invention include each of theabove-described features.

Alternative versions of the described features are also possible. Thus,there is no requirement that in all versions of the invention, thecutting device be the above-described circularly shaped disc. In someversions of the invention, the cutting device may be a blade likedevice. such a device may include a number of blade like arms thatproject outwardly from a center hub.

Also, while the above-described embodiment of the mill head of thisinvention has a use-once disposable mill head 34, other versions may bedesigned that have more sterilizable and reusable components. Thus, inone alternative version of this invention, the mill head has a bottomsection that is integral from the base. The mill head includes aremovable top. After use of this version of the invention, the mill topis removed. This allows access to the cutting device for eithersterilization or replacement. Separation of the two sections of the millalso makes it possible to access the interior portions of the mill forcleaning.

In some versions of the invention, the complementary geometric featuresthat facilitate the alignment of the cutting device with the drivespindle that rotates the cutting device may be different from what isdescribed above. Thus in some versions of the invention, an alignmentpin may project away from the cutting device. In these versions of theinvention, the drive spindle is formed with a bore positioned to receivethe pin. The bore may be conical in profile such that as the pin entersthe bore the pin as well as the whole of the cutting device, is centeredrelative the drive spindle.

Likewise, the configuration of the of the complementary couplingfeatures on the cutting device and drive spindle that transfer torque tothe cutting device may vary from what has been described. In someversions of the invention, teeth may project from the cutting device.These teeth engage in complementary slots or openings associated withthe drive spindle.

The structure of the fixed features of the base 32 and removable mill 34that inhibit rotation of the base are also not limited to what has beendisclosed. For example, small fingers may protrude from the mill. Inthese versions of the invention, the base 32 is provided with slots forreceiving the fingers. In some versions of the invention theanti-rotation pin that prevents rotation of the gear train housing 136may be a pin that extends into a bore that extends radially into thehousing. In some versions of the invention, either one or both of theplunger 40 and catch tray 42 have structural features that prevent theirunintended removal from the rest of the mill head 34. These features maybe detents or notches for receiving detents. If the feature is a detent,this member seats in a slot formed in the head 34. If the feature is anotch for receiving a detent, the detent is a static component locatedat a complementary location elsewhere on the head.

Alternative assemblies for releasably securing the mill head 34 to thebase 32 may likewise be provided. One such assembly may include a set ofrestraining arms that move in a horizontal plane to engage and releasefrom the mill head. In some versions of the invention, the release armsare attached to the mill head.

Likewise, some versions of the invention may have different assembliesfor releasably holding the mill head 34 to the base 32 and blocking themill from rotation. For example, in one alternative version of theinvention one of the base or mill is provided with L-shaped tabs. Theother of the mill or base is provided with key-hole type slots forreceiving the tabs. Once the mill is seated and rotated, the engagementof the tabs in the slots prevent mill removal. In these versions of theinvention, retractable teeth integral with the base may be extended toseat in slots formed in the mill. These teeth inhibit mill rotation.

Alternative structures of the cutting disc are also possible. Whiletypically not useful, a disc could have just a single opening-definingcutting edge. A disc with one or just a few openings may be desirablefor certain slow precision processes for forming bone chips. Further, insome versions of the invention, the disc may be formed so that theopening-defining edge against which the bone impinges and that forcesits separation from the bone stock is not located above the disc topsurface.

In some embodiments of the invention, a hand crank is attached to themill head. This hand crank is connected to the cutting device to rotatethe cutting device. An advantage of this version of the invention isthat it eliminates the need to provide the motor. In some versions ofthis embodiment of the invention a gear assembly connects the hand crankto the motor. This arrangement allows the user to with one hand, pushthe plunger downwardly so that the bone is pressed against the cuttingdevice while the other hand is used to turn the crank.

Therefore, it is the goal of the appended claims to cover all suchmodifications and variations that come within the rue spirit and scopeof this invention.

What is claimed is:
 1. A bone mill, said bone mill including: a millhead, said mill head including: a shell, said shell having: opposed andspaced apart top and bottom sections; a longitudinal axis that extendstop-to-bottom through said shell; a first opening in the shell topsection; a second opening in the shell bottom section; and a thirdopening in the shell bottom section, the third opening being closer tothe longitudinal axis of the shell than the second opening; a cuttingdevice movably disposed in said shell between the top and bottomsections, said cutting device including: at least one cutting featurelocated between the shell first and second openings configured to cutbone stock introduced into the shell first opening into bone chips sothat the bone chips are discharged through the shell second opening; anda coupling feature that is accessible through the shell third opening;and a barrier disposed in said shell around the third opening so as toseparate the second and third openings and prevent the discharge of bonechips through the third opening; a base, said base separate from saidmill head and having a surface for receiving said mill head shell; amotor disposed in the base; a drive stem connected to said motor so asto rotate upon actuation of said motor; a spindle connected to saiddrive stem to both rotate with said drive stem to and, while rotatingwith said drive stem, move longitudinally relative to said drive stem,said spindle including a coupling feature shaped to engage said cuttingdevice coupling feature across the mill head shell third opening sothat, when said coupling features engage, said cutting device isactuated upon the rotation of said spindle and wherein said spindle islocated between said motor and said mill head cutting device; a biasingmember connected to said spindle that exerts a force on said spindle soas to urge said spindle away from said motor and said spindle couplingfeature into engagement with said cutting device coupling feature assaid spindle is rotated by said motor; and a locking assembly integralwith at least one of said mill head or said base for releasably holdingsaid shell to the base in opposition to the force exerted by saidbiasing member.
 2. The bone mill of claim 1, wherein, said mill headfurther includes a catch tray removably attached to said shell adjacentthe shell second opening.
 3. The bone mill of claim 1, wherein saidbiasing member is a spring.
 4. The bone mill of claim 1, wherein saidlocking assembly includes at least one member that is moveably attachedto said base or said mill head shell that is shaped and positioned toreleasably engage the other of said mill head shell or said base.
 5. Thebone mill of claim 1, wherein: said cutting device is disposed in saidmill head shell so as to be moveable along or around at least two axes;and said spindle includes an alignment feature shaped to engage acomplementary portion of said cutting device, so that the engagement ofsaid alignment feature with said cutting device positions said cuttingdevice relative to said spindle so that said cutting device couplingfeature is positioned to engage with said spindle coupling feature. 6.The bone mill of claim 5, wherein said spindle alignment feature isseparate from said spindle coupling feature.
 7. The bone mill of claim1, wherein said cutting device is a disc that is rotatably mounted insaid shell.
 8. The bone mill of claim 1, wherein: said cutting device isformed with at least one opening, the cutting device at least oneopening functioning as said cutting device coupling feature; and saidspindle coupling feature is at least one tooth that projects from asurface of said spindle to engage in the at least one opening formed insaid cutting device.
 9. A bone mill, said bone mill including: a base,said base having a receiving surface; a motor disposed in said base; adrive stem attached to said motor to rotate upon actuation of saidmotor; a spindle connected to said drive stem to rotate with said drivestem and, while rotating with said drive stem, move longitudinallyrelative to said drive stem, said spindle having a coupling feature andwherein said spindle is positioned adjacent the base receiving surface;a mill head, said mill head being separate from and being removablyattachable to said base, said mill head including: a shell, said shellshaped to seat on the receiving surface of said base and having: opposedand spaced apart top and bottom sections; a longitudinal axis thatextends top-to-bottom through said shell; a first opening in the shelltop section; second and third openings in the shell bottom section, thesecond opening being spaced further from the shell longitudinal axisthan the third opening; a cutting device movably disposed in said shell,said cutting device including: at least one cutting feature locatedbetween the shell first and second openings configured to cut bone stockintroduced into the shell first opening into bone chips so that the bonechips are discharged through the shell second opening; and a couplingfeature that is accessible through the shell third opening and that isconfigured to engage said spindle coupling feature so that theengagement of said coupling feature results in said spindle, whenrotating, actuating said cutting device; and a barrier disposed in shellaround the third opening so as to be located between the third openingand said cutting device at least one cutting feature so as to preventthe discharge of bone chips through the third opening and to separatethe second and third shell openings from each other; a first biasingmember connected to said spindle that urges said spindle away from saidmotor so that when said mill head shell is disposed on the basereceiving surface, said spindle coupling feature is urged intoengagement with said cutting device coupling feature as said spindle isrotated; and at least one locking arm movably attached to said base soas to have a first position in which said locking arm holds said millhead shell to said base and a second position in which said locking armis spaced from said mill head shell.
 10. The bone mill of claim 9,further including a second biasing member that extends between said baseand said at least one locking arm that holds said locking arm in thefirst position.
 11. The bone mill of claim 9, wherein said mill headcutting device is a disc that is rotatably mounted in said mill headshell.
 12. The bone mill of claim 9, wherein: said cutting device isdisposed in said mill head shell so as to be moveable along or around atleast two axes; and said spindle includes an alignment feature shaped toengage a complementary portion of said cutting device, so that theengagement of said alignment feature with said cutting device positionssaid cutting device relative to said spindle so that said cutting devicecoupling feature is positioned to engage with said spindle couplingfeature.
 13. The bone mill of claim 9, wherein: said spindle has alongitudinal axis around which said spindle, when actuated by saidmotor, rotates and said spindle coupling feature consists of at leastone tooth that is located radially outwardly of the longitudinal axis ofsaid spindle; and said cutting device coupling feature is at least oneopening in said cutting device that is shaped to receive the said atleast one tooth of said spindle.
 14. The bone mill of claim 9, wherein:said spindle coupling feature is at least one tooth that projects from asurface of said spindle; and said cutting device is formed with at leastone opening that is dimensioned to receive said at least one tooth ofsaid spindle so that the at least one opening functions as said cuttingdevice coupling feature.
 15. A bone mill, said bone mill including: amill head, said mill head including: a shell, said shell having: opposedtop and bottom sections; a longitudinal axis that extends top-to-bottomthrough said shell; a first opening in the shell top section; a secondopening in the shell bottom section; and a third opening in the shellbottom section, the third opening being closer to the longitudinal axisthan the second opening; and a cutting device movably disposed in saidshell, wherein said cutting device is able to rotate or move within saidshell around at least two axes and said cutting device includes: atleast one cutting feature located between the shell first and secondopenings, said cutting feature configured to cut bone stock introducedinto the shell first opening into bone chips that are discharged throughthe shell second opening; an alignment feature accessible through theshell third opening; and a coupling feature accessible through the shellthird opening; and a barrier disposed in said shell around the thirdopening so as to separate the cutting device at least one cuttingfeature from the third opening and to separate the second and thirdshell openings from each other; and a base, said base separate from saidmill head and having a surface for receiving said mill head shell; amotor disposed in the base; a drive stem connected to said motor so asto rotate upon actuation of said motor; a spindle connected to saiddrive stem to both rotate with said drive stem to and, while rotatingwith said drive stem, move longitudinally relative to said drive stem,said spindle including a coupling feature shaped to engage said cuttingdevice coupling feature so that when said coupling features engage, saidcutting device is actuated upon the rotation of said spindle and whereinsaid spindle is located between said motor and said mill head shell; abiasing member connected to said spindle that exerts a force on saidspindle so as to urge said spindle away from said motor and spindlecoupling feature towards said cutting device coupling feature as saidspindle is rotated; an alignment feature attached to said spindle, saidspindle alignment feature shaped to engage said cutting device alignmentfeature so that, as a consequence of the engagement of said alignmentfeatures, said cutting device is positioned relative to said spindle sothat said cutting device coupling feature is positioned to engage saidspindle coupling feature; and a locking assembly integral with at leastone of said mill head or said base for releasably holding said shell tosaid base in opposition to the force exerted by said biasing member. 16.The bone mill of claim 15, wherein said locking assembly includes atleast one member that is moveably attached to said base or said millhead shell that is shaped and positioned to releasably engage the otherof said mill head shell or said base.
 17. The bone mill of claim 15,wherein: said spindle alignment feature projects outwardly from saidspindle; and said cutting device alignment feature is an opening formedin said cutting device that is dimensioned to receive said spindlealignment feature.
 18. The bone mill of claim 15, wherein: said spindlecoupling feature and said spindle alignment features are separatefeatures integral with said spindle; and said cutting device couplingfeature and said cutting device alignment feature are separate featuresformed with said cutting device.
 19. The bone mill of claim 15, wherein,said mill head further includes a catch tray removably attached to saidshell adjacent the shell second opening.
 20. The bone mill of claim 15,wherein said biasing member is a spring.
 21. The bone mill of claim 15,wherein said cutting device is a disc that is rotatably mounted in saidshell.
 22. The bone mill of claim 15, wherein said barrier internal tosaid mill head shell consists of ring integral with the shell bottomsection that surrounds the shell third opening.
 23. The bone mill ofclaim 15, wherein: internal to said mill head shell is an impingementsurface; and said cutting device at least one cutting feature isconfigured to press bone stock against the impingement surface so thebone stock is sheared between said cutting device and the impingementsurface.
 24. The bone mill of claim 1, wherein said barrier internal tosaid mill head shell consists of ring integral with the shell bottomsection that surrounds the shell third opening.
 25. The bone mill ofclaim 1, wherein: internal to said mill head shell is an impingementsurface; and said cutting device at least one cutting feature isconfigured to press bone stock against the impingement surface so thebone stock is sheared between said cutting device and the impingementsurface.
 26. The bone mill of claim 9, wherein said barrier internal tosaid mill head shell consists of ring integral with the shell bottomsection that surrounds the shell third opening.
 27. The bone mill ofclaim 9, wherein: internal to said mill head shell is an impingementsurface; and said cutting device at least one cutting feature isconfigured to press bone stock against the impingement surface so thebone stock is sheared between said cutting device and the impingementsurface.